1. Field of the Invention
This invention relates to conductive polymer compositions which exhibit a positive temperature coefficient of resistivity and to electrical devices comprising said compositions.
2. Discussion of the Prior Art
Conductive polymer compositions containing particles dispersed in a polymer matrix are described in the art. The conductive particles commonly used are of carbon black. The particles are generally dispersed in crystalline thermoplastic polymers, elastomeric polymers, mixtures of one or more crystalline thermoplastic polymers with one or more elastomeric polymers, and thermosetting resins. Reference may be made, for example, to U.S. Pat. Nos. 3,823,217 (Kampe), 3,861,029 (Smith-Johannsen et al.), 3,950,604 (Penneck), and 4,177,376 (Horsma et al.) and to U.S. patent application Ser. Nos. 904,736 (Penneck et al.), 798,154 (Horsma), now abandoned, 899,658 (Blake et al.), 965,343 (Van Konynenburg et al.), now U.S. Pat. Nos. 4,237,441, 965,344 (Middleman et al.), now U.S. Pat. Nos. 4,238,812, 965,345 (Middleman et al.), now U.S. Pat. Nos. 4,242,573, 6,773 (Simon) now U.S. Pat. Nos. 4,255,698, and 75,413 (Van Konynenburg) now U.S. Pat. No. 4,304,987. The disclosures of these patents and applications are incorporated by reference herein.
Some of the conductive polymer compositions containing dispersed carbon black particles exhibit what is referred to as a positive temperature coefficient of resistance (PTC) and undergo a sharp increase in resistivity as their temperature rises above a particular value. This temperature is frequently referred to as the switching temperature or the anomaly temperature.
Conductive polymer compositions in which the conductive particles are metal powders, particles or flakes, are also disclosed in the art. These compositions generally have low resistivity, depending on the amount and characteristics of the metal particles incorporated into the polymer. Some of these compositions are reported to be PTC materials and their use in current limiting or current interrupting devices has been proposed. However, the use of these compositions is limited by internal arcing which can lead to catastrophic failure of the device and in some cases, complete burning of the device. In J. Phys. D: Appl. Phys. Vol. II, 17, Littlewood and Briggs report an investigation into the use of metal-filled epoxy resins in current interrupting devices. They report that damage due to internal arcing renders the device unsuitable for use at voltages greater than 10 volts.
In "Solid State Bistable Power Switch" by Shulman et al., National Aeronautics and Space Administration Report N68-35634 (1968), a study on a resettable fuse for high current applications is reported. The resettable fuse comprises metal particles dispersed in a polymer matrix comprising a silicone resin. It is reported that when a polyester material was used as the matrix, the device exploded after several successful trips. It was also found that in order for the fuse to be capable of being used at relatively high currents, the metal particles should be relatively large, about 20 mesh (about 850 microns). When smaller particles (325 mesh) were used in the device, high currents caused the particles to melt and fuse together. The resettable fuse of Shulman et al. indefinitely remains in the state into which it was last switched. Thus, when the device has tripped, that is, has switched into its high resistance state, it remains in that state until it is reset. To reset the device i.e., switch it back to its low resistance state, it must be subjected to a relatively high reset voltage pulse.
U.S. Pat. No. 3,983,075 (Marshall) discloses electrically conductive compositions comprising copper flakes dispersed in an epoxy resin binder. The compositions are used to make heaters. To improve uniformity between different batches of the conductive composition when the composition contains less than 50% by weight copper flake, carbon black in an amount of 5-10% by weight is added. The conductive compositions of Marshall are not PTC materials, as discussed in greater detail in the comparative example below. The U.S. Pat. No. 3,983,075 also reports that local overheating results in thermal degradation of the composition. The incorporation of carbon black is said to avoid local sparking by lowering the resistance between adjacent flakes.